SB-505124 is a selective inhibitor of transforming growth factor-beta type I receptors ALK4, ALK5, and ALK7

Clinically, there is a great need for small molecule inhibitors that could control pathogenic effects of transforming growth factor (TGF-beta) and/or modulate effects of TGF-beta in normal responses. Inhibition of TGF-beta signaling would be predicted to enhance re-epithelialization of cutaneous wounds and reduce scarring fibrosis. Selective small molecule inhibitors of the TGF-beta signaling pathway developed for therapeutics will also be powerful tools in experimentally dissecting this complex pathway, especially its cross-talk with other signaling pathways. In this study, we characterized 2-(5-benzo[1,3]dioxol-5-yl-2-tert-butyl-3H-imidazol-4-yl)-6-methylpyridine hydrochloride (SB-505124), a member of a new class of small molecule inhibitors related to imidazole inhibitors of p38, which inhibit the TGF-beta type I receptor serine/threonine kinase known as activin receptor-like kinase (ALK) 5. We demonstrate that this compound selectively and concentration-dependently inhibits ALK4-, ALK5-, and ALK 7-dependent activation of downstream cytoplasmic signal transducers, Smad2 and Smad3, and of TGF-beta-induced mitogen-activated protein kinase pathway components but does not alter ALK1, ALK2, ALK3 or ALK6-induced Smad signaling. SB-505124 also blocks more complex endpoints of TGF-beta action, as evidenced by its ability to abrogate cell death caused by TGF-beta1 treatment. SB-505124 is three to five times more potent than a related ALK5 inhibitor described previously, SB-431542.     

Inhibition of transforming growth factor (TGF)-beta1-induced extracellular matrix with a novel inhibitor of the TGF-beta type I receptor kinase activity: SB-431542

Transforming growth factor beta1 (TGF-beta1) is a potent fibrotic factor responsible for the synthesis of extracellular matrix. TGF-beta1 acts through the TGF-beta type I and type II receptors to activate intracellular mediators, such as Smad proteins, the p38 mitogen-activated protein kinase (MAPK), and the extracellular signal-regulated kinase pathway. We expressed the kinase domain of the TGF-beta type I receptor [activin receptor-like kinase (ALK)5] and the substrate, Smad3, and determined that SB-431542 is a selective inhibitor of Smad3 phosphorylation with an IC50 of 94 nM. It inhibited TGF-beta1-induced nuclear Smad3 localization. The p38 mitogen-activated protein kinase inhibitors SB-203580 and SB-202190 also inhibit phosphorylation of Smad3 by ALK5 with IC50 values of 6 and 3 microM, respectively. This suggests that these p38 MAPK inhibitors must be used at concentrations of less than 10 microM to selectively address p38 MAPK mechanisms. However, the p38 MAPK inhibitor SB-242235 did not inhibit ALK5. To evaluate the relative contribution of Smad signaling and p38 MAPK signaling in TGF-beta1-induced matrix production, the effect of SB-431542 was compared with that of SB-242235 in renal epithelial carcinoma A498 cells. All compounds inhibited TGF-beta1-induced fibronectin (FN) mRNA, indicating that FN synthesis is mediated in part via the p38 MAPK pathway. In contrast, SB-431542, but not the selective p38 MAPK inhibitor SB-242235, inhibited TGF-beta1-induced collagen Ialpha1 (col Ialpha1). These data indicate that some matrix markers that are stimulated by TGF-beta1 are mediated via the p38 MAPK pathway (i.e., FN), whereas others seem to be activated via ALK5 signaling independent of the p38 MAPK pathway (i.e., col Ialpha1).     

Inhibition of ALK5 as a new approach to treat liver fibrotic diseases

Liver fibrosis is the result of an unbalanced wound healing response to a chronic hepatic injury. Transforming growth factor-beta (TGF-beta) plays a major role in this process via the activation of hepatic stellate cells. Various approaches have been tested in animal models of fibrosis to block the effects of TGF-beta, including antibodies and soluble receptors. Here, we discuss the potential use of TGF-beta signaling inhibitors, acting at the TGF-beta type I receptor kinase (ALK5) level, as a possible therapy for liver fibrosis. Thus far, there is only one ALK5 inhibitor (GW6604) for which activity in models of liver fibrosis has been described, showing clear antifibrotic effects resulting in liver function improvement. However, due to the pleiotropic effects of TGF-beta, the beneficial antifibrotic effects of ALK5 inhibition should be carefully balanced against the potential risk of unwanted effects stemming from chronic treatment.     

ALK5 inhibition in renal disease

Recent advances have identified novel small molecule inhibitors of the transforming growth factor-beta (TGF-beta) type I receptor kinase as a potential therapy in organ remodeling diseases, such as chronic renal disease. Because TGF-beta is central to the progression of fibrosis, selective inhibition of this signaling pathway could provide a novel treatment in many fibrotic diseases. The rationale for inhibition of TGF-beta signaling in renal disease includes prevention of fibrosis, tubular dedifferentiation and vascular effects.     

Identification of novel inhibitors of the transforming growth factor beta1 (TGF-beta1) type 1 receptor (ALK5)

Screening of our internal compound collection for inhibitors of the transforming growth factor beta1 (TGF-beta1) type I receptor (ALK5) identified several hits. Optimization of the dihydropyrroloimidazole hit 2 by introduction of a 2-pyridine and 3,4-methylenedioxyphenyl group gave 7, a selective ALK5 inhibitor. With this information, optimization of the triarylimidazole hit 8 gave the selective inhibitor 14, which inhibits TGF-beta1-induced fibronectin mRNA formation while displaying no measurable cytotoxicity in the 48 h XTT assay.     

The ALK gene, an attractive target for inhibitor development

Anaplastic Lymphoma Kinase (ALK) is a receptor tyrosine kinase that belongs to the Insulin receptor subfamily involved as full length receptor in neural development. Even if the expression of ALK protein is down-regulated in the adults, the ALK full length is expressed in different types of tumors. Moreover, chromosomal rearrangements, involving the alk gene, can occur leading the formation of different ALK fusion proteins characterized by the kinase domain of ALK fused to several partners that determine cellular localization. Structural investigation and characterization of the ALK kinase domain in absence of its crystal structure constituted the basis of development of ALK small molecule inhibitors. Here, we described normal function of the ALK receptor and its role in tumors; formation of the constitutively activated ALK fusion proteins and we reported an update of developed small molecule inhibitors of the ALK kinase activity.     

Crizotinib, a small-molecule dual inhibitor of the c-Met and ALK receptor tyrosine kinases

Crizotinib (PF-02341066), under development by Pfizer, is an orally bioavailable, ATP-competitive, small-molecule inhibitor of the receptor tyrosine kinases (RTKs) c-Met (also known as hepatocyte growth factor receptor) and anaplastic lymphoma kinase (ALK), for the potential treatment of cancers dependent on these oncogenic kinases for growth and survival. Since the first published characterizations of crizotinib only a few years ago, the drug has been extensively validated as a highly specific inhibitor of c-Met and ALK among > 120 different RTKs surveyed. In preclinical tumor xenograft studies, crizotinib inhibited the growth and survival of cell lines dependent upon c-Met or ALK enzymatic activity. Crizotinib has been particularly effective against anaplastic large cell lymphoma and non-small cell lung cancer (NSCLC) cell lines that harbor ALK translocations resulting in expression of oncogenic ALK fusion proteins. During early-stage clinical testing, crizotinib was well tolerated and produced dramatic antitumor activity in patients with ALK-rearranged NSCLC. At the time of publication, an ongoing phase III clinical trial is comparing crizotinib with standard second-line chemotherapy in previously treated patients with NSCLC harboring ALK rearrangement, and a phase III trial comparing crizotinib with standard chemotherapy in the first-line setting in non-squamous lung cancer is planned. Thus, in the future, crizotinib is expected to become a highly used therapeutic for the treatment of ALK-rearranged tumors.     

Transforming growth factor-β type I receptor/ALK5 contributes to doxazosin-induced apoptosis in H9C2 cells

The mechanism of doxazosin-induced apoptosis through α₁-adrenoceptor-independent pathway has been reported in various types of cell models. However, the molecular events involved in this effect are still not fully discovered. In present study, we proposed that the transforming growth factor-β type I receptor (TβRI/ALK5) may contribute to the doxazosin-induced apoptosis in H9C2 cardiomyoblasts. Via the detection of cell viability, apoptotic nuclei, and caspase-3 activity, we found that doxazosin induced concentration- and time-dependent apoptosis in H9C2 cells. The cell apoptosis induced by 30 μM doxazosin was exacerbated by the addition of 10 ng/ml transforming growth factor-β1 (TGF-β1). Doxazosin or TGF-β1 alone respectively elevated p38 mitogen-activated protein kinases (MAPK) and Smad3 protein phosphorylation in H9C2 cells. However, the cotreatment of doxazosin and TGF-β1 attenuated the TGF-β1-induced Smad3 protein phosphorylation and increased doxazosin-induced p38 MAPK protein phosphorylation. Furthermore, inhibitors of TβRI/ALK5 (SB431542) and p38 MAPK (SB202190) or TβRI/ALK5 knockdown all dramatically reduced the doxazosin-induced apoptosis in H9C2 cells. In conclusion, our results demonstrated that TβRI/ALK5-p38 MAPK phosphorylation signaling pathway could contribute to doxazosin-induced cell apoptosis, which could be further enhanced by TGF-β1 in association with attenuating Smad3 phosphorylation in H9C2 cells.     

Signal transduction in wound pharmacology

Growth factors such as TGF-beta, PDGF and FGF are thought to play important roles in wound healing. However, their biological activity and signal transduction during wound repair remain poorly understood. Growth factors are often ligands for receptor tyrosine kinase and receptor serine/threonine kinases. With recent advances in signal transduction by receptor kinases, we are beginning to understand the underlying mechanism of how growth factors may regulate cutaneous wound repair. In this paper, we will describe the pharmacological effects of growth factors on wound healing, and discuss the potential underlying signaling mechanisms. Thus, we hope to provide the basis for designing more specific therapeutics for wound healing in the near future.     

Role of TGF-beta1 in vascular smooth muscle cell apoptosis induced by angiotensin II

Both Angiotensin II and transforming growth factor beta-1 (TGF-beta1) are important mediators of vascular smooth muscle cell function and have been reported to mediate the balance between proliferation and apoptosis. Some crosstalk between Angiotensin II and TGF-beta1 in end-organ hypertension has been established. However, whether TGF-beta1 is able to mediate Angiotensin II-induced vascular cell damage remains unknown. Vascular smooth muscle cells were obtained from rat thoracic aorta and cultured in 10% foetal calf serum. In all experiments, medium was changed to a low-serum (0.4% foetal calf serum) or serum-free one with or without Angiotensin II. Apoptosis was assessed by DNA fragmentation, DNA synthesis was measured as bromo-deoxyuridine uptake. TGF-beta1 production was determined by Enzyme-linked Immunosorbent Assay (ELISA) from cell conditioned media, RT-PCR from cell lysates and confocal immunostaining of fixed cells. Angiotensin II induced apoptosis in the absence of DNA synthesis when coincubated at 1 microM. Neither the specific anti-TGF-beta1 monoclonal antibody (50 microg/ml) nor the novel activin-like kinase (ALK)-4/5/7 synthetic inhibitor SB-431542 (4-(5-benzo(1,3)dioxol-5-yl-4-pyridin-2-yl-1H-imidazol-2-yl)benzamide) at 10 microM were able to inhibit this effect. Angiotensin II induced expression of TGF-beta1 without further secretion of this cytokine. This effect was not affected by incubation with the AT1 inhibitor irbesartan (10 microM). A pharmacological approach to TGF-beta1 inhibition would be unable to reverse the apoptotic effect of Angiotensin II on vascular smooth muscle cells.     

Inhibition of TGFβ type I receptor activity facilitates liver regeneration upon acute CCl4 intoxication in mice

Liver exhibits a remarkable maintenance of functional homeostasis in the presence of a variety of damaging toxic factors. Tissue regeneration involves cell replenishment and extracellular matrix remodeling. Key regulator of homeostasis is the transforming growth factor-β (TGFβ) cytokine. To understand the role of TGFβ during liver regeneration, we used the single-dose carbon tetrachloride (CCl₄) treatment in mice as a model of acute liver damage. We combined this with in vivo inhibition of the TGFβ pathway by a small molecule inhibitor, LY364947, which targets the TGFβ type I receptor kinase [activin receptor-like kinase 5 (ALK5)] in hepatocytes but not in activated stellate cells. Co-administration of LY364947 inhibitor and CCl₄ toxic agent resulted in enhanced liver regeneration; cell proliferation (measured by PCNA, phosphorylated histone 3, p21) levels were increased in CCl₄ + LY364947 versus CCl₄-treated mice. Recovery of CCl₄-metabolizing enzyme CYP2E1 expression in hepatocytes is enhanced 7 days after CCl₄ intoxication in the mice that received also the TGFβ inhibitor. In summary, a small molecule inhibitor that blocks ALK5 downstream signaling and halts the cytostatic role of TGFβ pathway results in increased cell regeneration and improved liver function during acute liver damage. Thus, in vivo ALK5 modulation offers insight into the role of TGFβ, not only in matrix remodeling and fibrosis, but also in cell regeneration.     

Adenosine A2A receptor occupancy stimulates collagen expression by hepatic stellate cells via pathways involving protein kinase A, Src, and extracellular signal-regulated kinases 1/2 signaling cascade or p38 mitogen-activated protein kinase signaling pathway

Prior studies indicate that adenosine and the adenosine A2A receptor play a role in hepatic fibrosis by a mechanism that has been proposed to involve direct stimulation of hepatic stellate cells (HSCs). The objective of this study was to determine whether primary hepatic stellate cells produce collagen in response to adenosine (via activation of adenosine A2A receptors) and to further determine the signaling mechanisms involved in adenosine A2A receptor-mediated promotion of collagen production. Cultured primary HSCs increase their collagen production after stimulation of the adenosine A2A receptor in a dose-dependent fashion. Likewise, LX-2 cells, a human HSC line, increases expression of procollagen alphaI and procollagen alphaIII mRNA and their translational proteins, collagen type I and type III, in response to pharmacological stimulation of adenosine A2A receptors. Based on the use of pharmacological inhibitors of signal transduction, adenosine A2A receptor-mediated stimulation of procollagen alphaI mRNA and collagen type I collagen expression were regulated by signal transduction involving protein kinase A, src, and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (erk), but surprisingly, adenosine A2A receptor-mediated stimulation of procollagen alphaIII mRNA and collagen type III protein expression depend on the activation of p38 mitogen-activated protein kinase (MAPK), findings confirmed by small interfering RNA-mediated knockdown of src, erk1, erk2, and p38 MAPK. These results indicate that adenosine A2A receptors signal for increased collagen production by multiple signaling pathways. These results provide strong evidence in support of the hypothesis that adenosine receptors promote hepatic fibrosis, at least in part, via direct stimulation of collagen expression and that signaling for collagen production proceeds via multiple pathways.     

N-acetyl-L-cysteine suppresses TGF-beta signaling at distinct molecular steps: the biochemical and biological efficacy of a multifunctional, antifibrotic drug

The interrelated signaling via TGF-beta1 and reactive oxygen species has a profound impact on fibrogenesis and is therefore selected as target for antifibrotic therapies. This prompted us to investigate the influence of the antioxidant N-acetyl-L-cysteine on TGF-beta signaling in culture-activated hepatic stellate cells, the most relevant pro-fibrogenic cell type in liver. Dissection of the molecular steps involved in TGF-beta signaling revealed that N-acetyl-L-cysteine dose-dependently abrogated the induction of the TGF-beta1 signaling reporter gene activation, the phosphorylation of Smad2 and Smad3, and the up-regulation of Smad7 mRNA. By means of Western blot analysis and cross-linking experiments, it was demonstrated that these effects are based on disintegration of TGF-beta1 and the TGF-beta receptor endoglin, as well as a reduced ligand binding capacity of betaglycan. We conclude that N-acetyl-L-cysteine is a specific inhibitor of TGF-beta signaling targeting different components of the TGF-beta signaling machinery. In conclusion, these findings suggest that this non-toxic aminothiol downregulates TGF-beta signal transduction thereby mediating beneficial effects on experimental liver fibrosis characterized by TGF-beta hyperactivity.     

Selective pharmacological inhibitors reveal differences between Thy-1- and T cell receptor-mediated signal transduction in mouse T lymphocytes

A compelling body of evidence suggests a role for Thy-1 (CD90), a cell surface glycoprotein of mouse T lymphocytes, in signal transduction resulting in T cell activation. Despite more than 3 decades of investigation, intracellular biochemical events governing the Thy-1 signaling cascade are only vaguely understood. We have employed selective pharmacological inhibitors of signaling molecules to compare downstream elements participating in the Thy-1 signal transduction pathway with those involved in the T cell receptor (TCR)/CD3-associated signaling pathway. Mitogenic anti-Thy-1 or anti-CD3 monoclonal antibody (mAb) were used to cause T cells from C57BL/6 mice to proliferate in the presence or absence of different pharmacological inhibitors. Cyclosporine A, herbimycin A, LY294002, calphostin C and PD98059 all inhibited anti-Thy-1-induced T lymphocyte proliferation, indicating the involvement of calcineurin, protein tyrosine kinases, phosphatidylinositol 3-kinase, protein kinase C, and MEK1 (MAPK kinase 1), respectively, in Thy-1 signaling. Similar results were obtained when T cells were stimulated through the TCR with anti-CD3 monoclonal antibody in the presence or absence of the different inhibitors. Interestingly, the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 augmented anti-Thy-1-induced T cell proliferation, whereas anti-CD3-induced proliferative response was partially suppressed by the same inhibitor. The Thy-1 signal transduction pathway, therefore, shares a requirement for calcineurin and several major kinase families with the TCR signaling pathway. However, Thy-1 and TCR-associated signaling pathways are differentially regulated by p38 MAPK.     

Signal transduction pathways of G protein-coupled receptors and their cross-talk with receptor tyrosine kinases: lessons from bradykinin signaling

G protein-coupled receptors (GPCRs) represent a major class of drug targets. Recent investigation of GPCR signaling has revealed interesting novel features of their signal transduction pathways which may be of great relevance to drug application and the development of novel drugs. Firstly, a single class of GPCRs such as the bradykinin type 2 receptor (B2R) may couple to different classes of G proteins in a cell-specific and time-dependent manner, resulting in simultaneous or consecutive initiation of different signaling chains. Secondly, the different signaling pathways emanating from one or several GPCRs exhibit extensive cross-talk, resulting in positive or negative signal modulation. Thirdly, GPCRs including B2R have the capacity for generation of mitogenic signals. GPCR-induced mitogenic signaling involves activation of the p44/p42 "mitogen activated protein kinases" (MAPK) and frequently "transactivation" of receptor tyrosine kinases (RTKs), an unrelated class of receptors for mitogenic polypeptides, via currently only partly understood pathways. Cytoplasmic tyrosine kinases and protein-tyrosine phosphatases (PTPs) which regulate RTK signaling are likely mediators of RTK transactivation in response to GPCRs. Finally, GPCR signaling is the subject of regulation by RTKs and other tyrosine kinases, including tyrosine phosphorylation of GPCRs itself, of G proteins, and of downstream molecules such as members of the protein kinase C family. In conclusion, known agonists of GPCRs are likely to have unexpected effects on RTK pathways and activators of signal-mediating enzymes previously thought to be exclusively linked to RTK activity such as tyrosine kinases or PTPs may be of much interest for modulating GPCR-mediated biological responses.